Implantable defibrillators use truncated exponential waveforms to defibrillate the heart. The earliest devices used monophasic waveforms. More recent clinical investigations have evaluated the increased effectiveness of biphasic waveforms. See Troup, Implantable Cardioverters and Defibrillators, Current Problems in Cardiology, Volume XIV, Number 12, Dec. 1989, pages 729-744. Some investigators have even recommended the use of triphasic waveforms as the most effective waveform for defibrillating a heart. See U.S. Pat. No. 4,637,397 issued to Jones and Jones on Jan. 20, 1987.
As described by Troup, monophasic waveforms are typically produced using silicon controlled rectifier (SCR) technology that truncates the pulse by "dumping" the energy on the defibrillator capacitor. This leaves no energy available on the capacitor for producing multiphasic waveforms. As further described by Troup, there have been two methods available for truncation of a monophasic defibrillation waveform. According to one method, pulse truncation is accomplished by comparing the capacitor voltage to a reference voltage which is usually chosen as a function of the waveform leading edge voltage. The result is a defibrillation pulse with a constant ratio of trailing edge to leading edge voltage, or a "constant tilt" pulse.
Defibrillation pulse "tilt", described as percent tilt, is defined as follows: EQU %Tilt=100[1-(V.sub.f /V.sub.i)]
where V.sub.f is the trailing edge voltage of the pulse and V.sub.i is the leading edge voltage.
According to the second method, the defibrillation pulse is truncated by a timing circuit so that the pulse duration is constant. Biphasic waveform generators have used MOS switches to produce the defibrillator output. The MOS switch technique is better suited to multiphasic waveforms since the defibrillator capacitor does not need to be "dumped" to truncate the pulse.
Prior art biphasic waveforms have been programmable in terms of pulse duration. The disadvantage of programming biphasic waveforms in terms of duration can be seen in FIG. 1. Panel 1 of FIG. 1 shows a conventional biphasic waveform with a 50 ohm load. Panel 2 shows a conventional biphasic waveform with the same duration of phases with a 25 ohm load. With a 50 ohm load, there is adequate residual voltage to produce an effective negative phase of the biphasic waveform. However, at the same pulse durations, with a 25 ohm load, the voltage during the positive phase has decayed to the point where very little is left for the negative phase.
While it is possible to select optimal pulse durations for a given patient impedance, the patient impedance may change. In particular, for higher defibrillation voltages, the patient impedance is lower. In addition, over time, the lead impedance may increase due to the build-up of scar tissue.
Due to their small size and battery operation, implantable defibrillators have limited output energy capability. It is not unusual for an implantable defibrillator to have only slightly more output capability than is required to defibrillate a patient. This lack of safety margin makes it all the more important that the output energy that is available is used in the most effective manner. While biphasic waveforms are a step in the right direction, the optimal settings for the positive and negative phase durations have not been addressed in the prior art.
U.S. Pat. No. 4,850,357 issued to Stanley M. Bach, Jr. on Jul. 25, 1989, discloses a circuit for generating a biphasic defibrillation waveform wherein both the positive and negative phases have constant tilt. However, the Bach, Jr. defibrillator generates a biphasic waveform having fixed characteristics. That is, only a single type of waveform can be delivered that has a first positive pulse having a specified constant tilt and a second negative pulse also having a specified constant tilt. Thus, the Bach defibrillator circuit provides none of the therapeutic flexibility that is desirable in restoring rhythm to a fibrillating heart.